1. Field of the Invention
The present invention relates to a thermoplastic resin injection molding machine, and more particularly to an injection molding machine of which the plasticating unit and the injecting unit are provided so as to be independent of each other.
2. Description of the Related Art
Conventionally, as an injection molding machine of which the plasticating unit for plasticating a thermoplastic resin and the injecting unit for injecting the plasticated resin into a metallic mold are provided so as to be independent of each other, screw replasticating type injection molding machines have been known. In the front of the injecting plunger of the injecting unit, a chamber, called a reservoir, for reserving a resin in an amount corresponding to one shot is formed. The resin plasticated by means of the plasticating unit is fed into the chamber, and injected into the mold by moving the injecting plunger forward.
In the case of such an injection molding machine, as the molding cycle, injection, dwelling, cooling, and mold-opening-closing are repeated in that order as shown in FIG. 1A. The plasticating unit is stopped during the injection, the dwelling, and the mold opening-closing. At the same time when the cooling process is started, the screw of the plasticating unit is driven so that the resin plasticated in the reservoir is fed, and synchronously, the injecting plunger is moved backward whereby the weighing is carried out.
The term xe2x80x9cmarginxe2x80x9d means an excess time-period till the mold opening-closing is started after the cooling is completed, and is provided for the stabilization of the molding cycle. The time-period is changed correspondingly to the shot capacity.
As seen in the above-description, in the case of injection molding machines using an injecting plunger, it is required to determine optimal values with respect to the weighing and a molding pressure for each injecting molding machine and moreover, each of the molds if they are different from each other. Accordingly, the optimal values are determined by repeating the trial shot. Thus, these injection molding machines have the fault that it takes a long time to carry out the work for determining the optimal values.
Moreover, for a conventional injection molding machine, it is necessary to provide a space for a reservoir and a plunger-stroke. Thus, there has been the fault that the size of the injection molding machine is large as a whole.
Accordingly, it is an object of the present invention to provide a thermoplastic resin injection molding machine of which the weighing process is obviated, with which simplified, high precision injection molding is enabled, and which can be reduced in size.
To achieve the above-described object, according to the present invention, there is provided a thermoplastic resin injection molding machine which comprises a plasticating unit for plasticating a thermoplastic resin, and an injecting unit connected to the plasticating unit through a connecting passage to inject the plasticated resin into a mold through an injection port, the injecting unit including a rotary pump of which the suction side is connected to the plasticating unit through the connecting passage and the discharge side is connected to the injection port, whereby the plasticated resin is intermittently injected through the injection port by means of the rotary pump.
According to the present invention, as the injecting unit, the rotary pump is used. Therefore, the plasticated resin is discharged in one direction only, and it is unnecessary to provide the process of moving-backward, that is, the weighing process. In particular, it is unnecessary to determine an optimal value with respect to the weighing before hand, in contrast to a conventional injection molding machine. The weighing can be adjusted so as to have an optimal value only by controlling the rotation of the rotary pump. Thus, the work for repeating the trial shot to determine the optimal value can be obviated.
Further, in the case of the rotary pump, a motor, a rotary actuator, or the like may be used as a driving source. Accordingly, the driving source can be reduced in size as compared with a plunger which is reciprocated, and the necessary space can be decreased. Therefore, as a whole, the injection molding machine can be reduced in size.
As the rotary pump of the present invention, different types of rotary pumps such as a circumscribing gear pump, an inscribing gear pump, a trochoid pump, a vane pump, a screw pump, or the like can be employed. For example, when a circumscribing gear pump is used, desirably, a relatively high injection pressure can be attained, and moreover, an injection-performance is stable even at a high temperature.
For the purpose of attaining molding products of high quality, it is necessary to control the shot capacity and the injection pressure at a high precision. Conventionally, it has been necessary to determined optimal values with respect to the weighing and the injection pressure by trial shot. According to the present invention, the shot capacity can be continuously adjusted by the rotation of the rotary pump. Accordingly, molding products of high quality can be obtained by stopping the pump when the injection pressure reaches a predetermined value.
Thus, preferably, a pressure sensor is provided to detect an injection pressure in the mold, and the rotary pump is stopped when the injection pressure reaches a predetermined value. By controlling like this, the injection pressure in the mold can be controlled at a high precision, and molding products stable in qualities can be obtained.
Further, it is possible to control the injection pressure so as to have a predetermined value by no use of a pressure sensor. That is, a torque limiter may be provided on a driving shaft for driving the rotary pump to provide a slide when the torque exceeds a predetermined value. In this case, when the injection pressure exceeds a predetermined value, the torque limiter provides a slide. Therefore, even if the driving shaft of the rotary pump continues to be rotated, the injection pressure can be kept at a predetermined value. Hereupon, desirably, when the torque limiter provides a slide, the injection pressure is prevented from decreasing. As the torque limiter, means having the same function as a sliding clutch or the like are included.
When the plastication is carried out intermittently as in a conventional thermoplastic resin-injection molding machine (FIG. 1A), the plastication efficiency is ready to be reduced. Especially, for a resin having low engaging properties such as LCP, the reduction of the efficiency is remarkable.
On the other hand, according to the present invention, the weighing process is unnecessary. Therefore, it is not needed that the plastication process is synchronized with the weighing process. The plastication process can be converted to be an off-line as shown in FIG. 1B. That is, the plastication unit can be driven continuously during all the period of the molding cycle, and the plastication efficiency can be considerably enhanced. When the plastication is converted to be an off-line like this, the margin after the cooling, process is completed is unnecessary. Thus, the molding cycle can be significantly shortened as compared with the conventional one.
As described above, when the plastication unit is continuously driven, the pulsation of a resin is generated, caused by the intermittent injection of the rotary pump and the continuous supply by the plastication unit, so that stable injection can not be achieved. Accordingly, preferably, the connecting passage has the volume at which the pulsation of the resin, caused by the intermittent injection by the rotary pump and the continuous supply by the plasticating unit, can be absorbed. For example, stable injection is enabled by setting the volume of the connecting passage to be at least 100 times of the volume of the shot capacity. The qualities of molding products are enhanced.
For the purpose of absorbing the pulsation of the resin, caused by the intermittent injection by the rotary pump and the continuous supply by the plastication unit, preferably, in the connecting passage, disposed is an accumulator for reserving the resin plasticated with the plasticating unit, in the amount equal to or more than the shot capacity, and feeding the resin into the injecting unit. The accumulator has a simple structure, and is capable of reserving a large quantity of resin. Accordingly, the pulsation can be assuredly absorbed even if the shot capacity of the pump is increased.
Preferably, the accumulator is contained in the plasticating unit. That is, the plasticating unit comprises a cylinder, a screw disposed in the cylinder rotatably and movably in the axial direction, driving means for rotationally driving the screw, an accumulator chamber formed between the tip of the screw and the cylinder and being capable of reserving the plasticated resin in the amount equal to or more than the shot capacity, and actuating means for actuating the screw forward in the axial direction and feeding the resin in the accumulator, chamber to the injection unit. In this case, since the accumulator can be integrated with the plasticating unit in addition to the above-described advantage, that is, the assured absorption of the pulsation, the injection molding machine can be reduced in size. Moreover, since the plasticated resin does not stay in the accumulator chamber and is fed to the injecting unit sequentially, the deterioration of the resin is reduced. As the actuating means, a spring may be employed, or the resin in the accumulator chamber may be fed to the injecting unit by means of a pneumatic, hydraulic, or electric actuator.
According to the present invention, the rotary pump, the plasticating unit, and means for absorbing the pulsation of resin may be combined. In this case, the molding cycle can be shortened. Therefore, desirably, the present invention is applied to an injection molding machine of which the shot capacity is small (for example, 5 milliliter or less), that is, an injection molding machine of which the products quantity in a mold is small.
For the purpose of enhancing the productivity of injection molding, available are the following two methods. That is, the products quantity in a mold is increased, or the molding cycle is shortened. As to a conventional molding machine, it is necessary to synchronize the plastication with the weighing. Accordingly, the plastication can not be converted to an off-line, and the shortening of the molding cycle has a limitation. Accordingly, it has been dominant that the productivity is enhanced by increasing the products quantity in a mold, that is, by using a large-size mold. However, in the case of this method, the depreciation expense of the mold is high. In production of many kinds of products in small lots, which has been increasingly employed in recent years, there is caused the problem that the unit price of molding products is increased. Moreover, there has been the fault that since the injection pressure is increased, the required clamping force is high, and the clamping mechanism is large in size.
On the contrary, when the injection molding machine is employed with which the molding cycle is shortened, though the products quantity in a mold is small, the productivity comparable to that of a conventional injection molding machine can be assured though the mold is small in size, so that the depreciation expense of the mold can be significantly reduced. In addition, since a small-size mold having a small products-quantity in a mold is used, the injection pressure can be reduced, so that a sufficient injection pressure can be attained even by the rotary pump. Further, advantageously, the required clamping power is reduced, so that the mold-clamping mechanism, that is, the molding machine itself can be reduced in size, and the molding machine itself can be inexpensively formed.
Preferably, a nozzle is provided for the injection port of the injecting unit, and after the resin in the mold is cooled to be solidified, the rotary pump is driven reversely so that the residual pressure of the resin remaining the nozzle is removed. In this case, the resin in the nozzle can be prevented from leaking out when the mold is opened or closed.
Also preferably, a nozzle is provided for the injection port of the injecting unit, and a valve is provided to open or close the nozzle. In this case, the resin remaining in the nozzle can be also prevented from leaking out when the mold is opened or closed.